Lambda expression returning null android

问题:

I'm writing a lambda expression to convert the given latitude and longitude into an address. The expression is supposed to take co-ordinates as arguments and returns their corresponding address. However, the value returned is null. Following is my class:

public class LambdaDeclarations {

String loc;

private static final String TAG = "LambdaDeclarations";

public CoordinatesToAddressInterface convert = (latitude, longitude, context) -> {
    RequestQueue queue = Volley.newRequestQueue(context);

    Log.d(TAG, "onCreate: Requesting: Lat: "+latitude+" Lon: "+longitude);
    String url ="https://maps.googleapis.com/maps/api/distancematrix/json?units=metric&origins="+latitude+","+longitude+"&destinations="+latitude+","+longitude+"&key=AIzaSyCdKSW0glin4h9sGYa_3hj0L83zI0NsNRo";
    // Request a string response from the provided URL.
    StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
            (String response) -> {
                try {
                    JSONObject jsonObject = new JSONObject(response);
                    JSONArray destinations = jsonObject.getJSONArray("destination_addresses");
                    Log.d(TAG, "GETRequest: JSON Object: "+destinations.toString());
                    String location = destinations.toString();
                    Log.d(TAG, "Location: "+location);
                    setLocation(location);
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }, error -> Log.d(TAG, "onErrorResponse: That didn't work!"));
    queue.add(stringRequest);
    return getLocation();
};


public String getLocation() {
    return loc;
}

public void setLocation(String location) {
    this.loc = location;
    }
}

Following is the output from logcat:

09-16 10:31:09.160 26525-26525/com.rmit.tejas.mad_foodtruck_2 D/LambdaDeclarations: GETRequest: JSON Object: ["77 State Route 32, West Melbourne VIC 3003, Australia"]
Location: ["77 State Route 32, West Melbourne VIC 3003, Australia"]
09-16 10:31:09.176 26525-26525/com.rmit.tejas.mad_foodtruck_2 D/LambdaDeclarations: GETRequest: JSON Object: ["111 Adderley St, West Melbourne VIC 3003, Australia"]
Location: ["111 Adderley St, West Melbourne VIC 3003, Australia"]
09-16 10:31:09.177 26525-26525/com.rmit.tejas.mad_foodtruck_2 D/LambdaDeclarations: GETRequest: JSON Object: ["4\/326 William St, Melbourne VIC 3000, Australia"]
Location: ["4\/326 William St, Melbourne VIC 3000, Australia"]

Following is my usage:

myViewHolder.textView3.setText("Location: i->"+i+" add: "+l.convert.toAddress(trackingInfos.get(i).getLatitude(),trackingInfos.get(i).getLongitude(),context));

l is an object of the class LambdaDeclarations and following is the relevant interface:

public interface CoordinatesToAddressInterface {
String toAddress(double latitude, double longitude, Context context);
}

When I try to print the coordinates from the relevant adapter they are getting printed correctly. So the location is getting set properly but when I try to access it from another class it shows me a null value for the string. Can you please advise an alternate method to extract the location from the expression?

回答1:

First of all, Lambda Expression is just a anonymous class implementation, it was design to be used as a method or class argument and solve shadowing issues of anonymous class.
So in your case, you don't need it at all, just simply implement CoordinatesToAddressInterface interface as named class as usual.

Second, you used Volley wrong, the first lambda you provided to StringRequest, hereafter will be call response callback, is going to be called when HTTP request finish but the return statement

return getLocation();

will return null immediately before your setLocation(location) or even your response callback ever get executed, that why you got null every time you call convert(), though you can still see log that you print because the response callback will be executed anyway (assume that request is success).

To use response callback correctly, you have to update your UI inside callback, pretty much like this

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
public static final String TAG = "MyAdapter";
private RequestQueue mQueue;

public MyAdapter(Context context) {
    this.mQueue = Volley.newRequestQueue(context);
}

public RequestQueue getMyAdapterRequestQueue() {
    return this.mQueue;
}

    ...

@Override
public void onBindViewHolder(@NonNull final MyViewHolder holder, int position) {
    String url ="some url";

    StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
            (String response) -> {
                try {
                    JSONObject jsonObject = new JSONObject(response);
                    JSONArray destinations = jsonObject.getJSONArray("destination_addresses");
                    Log.d(TAG, "GETRequest: JSON Object: "+destinations.toString());
                    String location = destinations.toString();
                    Log.d(TAG, "Location: "+location);
                    // update UI
                    holder.mTextView.setText(location);
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }, error -> Log.d(TAG, "onErrorResponse: That didn't work!"));

    stringRequest.setTag(TAG);
    mQueue.add(stringRequest);
}

Of course, you can edit your interface's method signature and make your adapter implement that interface (I would rather do it this way though) but the point is that you have to process asynchronous results in callback method, never expect asynchronous operation's callback to finish before your next lines of code.

RequestQueue shouldn't be created per-request since it manages internal state that help you make request faster (caching), you can also cancel requests too in an event like phone rotation and your will get destroy, in this case, just call cancel method in Activity/Fragment's onStop()

@Override
protected void onStop () {
    super.onStop();
    if (myAdapter.getMyAdapterRequestQueue() != null) {
        myAdapter.getMyAdapterRequestQueue().cancelAll(MyAdapter.TAG);
    }
}

Response callback won't be called after you cancel request.